The invention relates to a method and an apparatus for controlling speed. More precisely, the invention relates to speed control of a motor vehicle during a hill descent.
Motor vehicles negotiating hill descents have to be braked during the descent in accordance with the speed to be observed and the steepness of the downhill gradient. Within certain limits it is mostly possible, by selecting a suitable gear speed, to utilize the braking torque of the drive motor to reduce the vehicle speed during a hill descent.
By means of a drive motor operating in overrunning mode, however, the variation of the speed during a descent may be achieved only imprecisely and to a slight extent. In order for example to slightly increase the descent speed, the driver may accelerate a little and drive the engine in part-load operation. However, this entails an unnecessary consumption of fuel as well as an unavoidable acoustic environmental pollution by the engine. In terms of engine wear too, it is not ideal to negotiate longer hill descents in part-load operation. If, conversely, the descent speed is to be slightly reduced, then the driver will actuate the service brake for this purpose.
Longer hill descents with an actuated service brake are for example customary in off-road vehicles that travel down steep inclines. If the steepness of the route changes, then the driver is requested to correct his brake actuation accordingly.
Devices, which on a downhill run keep the vehicle at a constant speed preset by the driver through an automatic actuation of the brakes, are already in use. Such control devices, which influence a vehicle service brake in a way that enables a hill descent at a constant speed, are known by the name of HDC (hill descent control) devices. HDC devices frequently make use of components that are already fitted as standard in the motor vehicle within the framework of for example antilock braking systems (ABS) or traction control systems (ASR). Such components include control units, wheel rotational speed sensors, electrically driven pumps and clutches, magnetic control valves, diverse sensors as well as further units and devices.
In many HDC devices, only the difference between a setpoint speed and an actual speed is determined, from which any required brake actuation is then derived. Other HDC devices determine a value for the road inclination and from this determine an actuating signal for the service brake. From WO 01/14185 A1 it is known for example to compensate the influence of the downhill road gradient in travelling direction upon the vehicle speed independently of “conventional” speed control in that the brake pressure determined by the speed control is increased by a compensating brake pressure, the amount of which is determined from the road inclination. The service brake is then acted upon by the sum of these two brake pressure components.
It has however been shown that such systems do not cope well with road sections of varying inclination. This manifests itself for example in control variations, unexpectedly strong exertions of influence on the service brake or a non-uniform vehicle speed in the transition between sections of differing inclination. Occasionally, it is also observed that the vehicle speed actually achieved by known HDC devices is not fully independent of the degree of the downhill gradient.
The underlying object of the invention is therefore to improve the control response and the operational reliability of the described speed control for motor vehicles.